1. Field of the Invention
The present invention relates to an image processing device and image processing method for converting a color image into a binary black and white image.
2. Description of the Related Art
Recently facsimile machines and copying machines (photocopiers) capable of scanning color images have been developed, and demand for these machines has increased along with the spread of high quality color printers and displays.
These image processing devices capable of processing color images utilize an optical color sensor to scan in an image in the three primary colors (RGB). However, if an associated printer or display is set to a black-and-white output mode or is not provided with a color output or display function, the scanned color image is converted to a black and white image based on lightness of the original color image.
FIG. 4 of the accompanying drawings illustrates processing steps used in such a conversion. Here, image data scanned-in in the three primary colors (RGB) is converted into multi-bit-depth color data according to L*a*b* color, one of the CIE (International Council on Illumination) standard model color systems, and then further converted to binary image data.
Referring to FIG. 4, the RGB image data scanned in by a scanner Sc is converted to CIE LAB color-system data using an RGB-Lab converter 100. Next, lightness L data (multiple bit-depth data) is converted to binary data B&W using a dithering unit 101, and output from a printer Pr. The dithering unit 101 has only one dither pattern. The values inside the parentheses indicate the bit number for each data.
The dithering process divides an image into a plurality of small areas, and changes the density of the black pixels in each area to provide an intermediary gray-scale image having a plurality of pixel density levels as a whole. Specifically, an area defined by n.times.n pixels (a dot matrix) is used as a unit of gradation (or an original image is divided into a plurality of matrices), and a dither pattern in which the threshold values between white and black are dispersed is applied to each matrix to obtain a binary image. In the dithering process, the lightness L of each pixel in the matrix is compared with a corresponding threshold value of the dither pattern; the pixel is converted to white if the lightness of the pixel is brighter than the threshold, and the pixel is converted to black if its lightness is darker than the threshold.
FIG. 5 of the accompanying drawings illustrates the color spacing according to the Lab color system. In the Lab color system, lightness is represented as L and chromaticity, showing hue and saturation, is represented as "a" and "b". Saturation "c" is given by (a.sup.2 +b.sup.2).sup.0.5, and hue "h" is given by tan.sup.-1 (b/a). Further, in the color spacing of the Lab color system, "+a" represents red levels, "-a" represents the green level, "+b" represents the yellow level, and "-b" represents the blue level. As the color values extend to the limits of each range, the colors become more vivid, and as the values approach the center, the colors become softer.
In a conventional image processing system, even when the chromaticity "a", "b" of dot matrices in a single original image differ, the matrices are converted into the same bilevel black and white data as long as their lightness L is the same. Therefore, the black and white image that is output upon color-to-monochrome conversion can look unnatural in comparison to the original color image.